yes, u can be really skinny and get a six pack because there are little fat surrounding that abdominal area. Humans "naturally" have a six-pack but it is just buried under layers of fat. Other people who arent skinny such as Arnold Schwartznegger (cant spell), male models etc, get six-packs because they have increased their abdominal muscles' size through certain exercises, so it shows up just like how some people do dumbells alot and get larger biceps.
With regards to the types of exercises required to get a six-pack - I'm not sure LOL. *I think* skinny girls don't get a six-pack because they actually ironically enough have enough fat in that area, taking into account their height/size and food consumption is less than men.
On another note, athletic/sporty men with an average waist can get six-packs *without* doing any type of exercise that can purposely increase their ab. muscles - which is a bit bizzarre, but I think perhaps their exercise from sporty activities impartially reflects onto their ab. muscles?